The invention relates to multi-track recording and reproducing apparatus for magnetic tape and particularly to such apparatus for stereo recording and reproducing in commercial radio broadcasting where it may be required to operate continuously with the highest possible standards of reliability and durability.
The process of tape recording stereo signals for subsequent replay requires that very close control be maintained on the time or phase relationship between the two channels. Even closer control is required in recording and reproducing more than two channels. If two microphones provide input to a stereo recorder while making a master recording, and the tape is not held in perfect alignment or the heads are not perfectly aligned when this master recording is played back later, one channel may play back slightly ahead of the other. If this stereo signal is mixed and played back in a mono system, or if the stereo signal is broadcast in the conventional stereo manner and received in mono, the right and left channels will be electronically mixed. Cancellation of certain signals will occur in this situation. Depending upon the amount of misalignment during playback, signals of some frequencies may be 180.degree. out of phase and will therefore cancel. Signals of other frequencies will be cancelled or distorted to varying extents. The relationship between frequency or wave length and the amount of cancellation will determine which frequencies will be cancelled or lowered in level.
This phase differential is of particular concern in a recorder-reproducer handling the standard National Association of Broadcasters cartridge. In a cartridge machine, the size and location of the tape guides are limited by the mechanical structure of the cartridge so there are no long tape paths and ample space for precision tape guidance as in reel-to-reel machines. Tape guidance in a cartridge machine is further deteriorated by the fact that the tape, being an endless loop, pulls off the center of the hub which is in a different plane than the normal tape running path and is pulled around a post in an effort to realign it before it passes the recording and reproducing heads. These considerations along with the fact that the cartridge case is molded of plastic parts which lack precision make a cartridge prone to phase cancellation or distortion. In a cartridge machine, if the phase error caused by tape skew is corrected during the recording process to make up for these tracking errors, it can be played back subsequently with good performance.